Upstream and downstream of ran GTPase.

Among the Ras family, Ran is a unique small G protein. It does not have a lipid modification motif at the C-terminus to bind to the membrane, which is often observed within the Ras family. Ran may therefore interact with a wide range of proteins in various intracellular locations. This means that Ran could play many different roles like nucleocytoplasmic transport, microtubule assembly and so on. All of the Ran functions should be regulated by RanGEF and RanGAP. It is an interesting issue why RCC1, a RanGEF, is localized in the nucleus and RanGAP1/Ran1p in the cytoplasm. It is possible that RCC1 checks the state of chromosomal DNA replication and transfers it to the downstream events through Ran; thereby, RCC1 would be involved in coupling the spatial localization of cellular macromolecules with the cell cycle progression. In this context, Ran will be a very important cell cycle mediator. There is yet another G protein cascade, Gtr1-Gtr2, which interacts with the Ran cycle.